The invention relates to a protective film whose high transparency and reliable adhesion make it particularly suitable for the temporary protection of automobile glass and also other sensitive surfaces such as metals, plastics and surface coatings.
On the long journey traveled by export vehicles in particular from the manufacturing plant to the end user, starting with the assembly of the vehicle and continuing by way of its loading onto railcars, transportation by rail or road, unloading from the railcar, loading onto ships, the ship journey, unloading from the ship, transportation to the central storage facility, and transportation through to the local dealer, the new vehicle is exposed to diverse harmful influences. Repairs to paint and windows, especially the windshield, are the order of the day, since the customer demands a flawless new vehicle. This is a considerable cost factor for the manufacturers, and there is a great interest in reducing this expense.
Whereas minor paint damage can generally be removed by means of a small refinish, making good a scratch on an automobile window necessitates its complete replacement.
The replacement of a windshield is particularly expensive. In the case of a windshield installed by bonding with a water-reactive PU adhesive, the technique now used almost universally, the windshield is removed using a wire which cuts through the adhesive film. In the course of this operation, the windshield frame paint is often damaged, necessitating additional repairs. For the manufacturer, accordingly, there arise costs for the replacement windshield, for its removal and installation, and, possibly, for the repair of the paint damage caused during its replacement. In some cases, additionally, the delivery times are prolonged significantly, and customers are unhappy.
For a number of years already, paint surfaces of new vehicles, especially exposed surfaces, such as trunk lids, roof and engine hood, in particular, have been effectively masked off using self-adhesive films. They protect the finishes against slight injuries and soiling by media such as battery acid or fuel, fly rust and bird excrement. These films generally comprise unoriented polyolefin films of polyethylene, polypropylene or propylene-ethylene copolymers, and also mixtures of these, filled with TiO2 to improve the long-term stability. The adhesive composition used frequently comprises polyisobutylene or polyethylene-vinyl acetate.
The former are disclosed in EP 0 519 278 A2, where the adhesive composition is applied to a film which in turn is used to protect automobiles. The pressure-sensitive adhesive is based on polyisobutylene rubber, having a dynamic modulus of elasticity of from 2xc3x97105 to 7xc3x97106 dyn/cm2, corresponding in SI units to a figure of from 2xc3x97104 to 70xc3x97104 Pa, at 60xc2x0 C. In addition, the adhesive may have been blended with a silicone oil or with a low molecular mass acrylic polymer.
DE 196 35 704 A1 describes a self-adhesive surface protection film made from polyolefins with an adhesive comprising polyethylene-vinyl acetate (EVA) with a vinyl acetate content of from 40 to 80 mol %, especially 70 mol %, and with a loss angle tan xcex4 of from 0.6 to 1.0, measured at a temperature of 60xc2x0 C. and a frequency of 10xe2x88x922 Hz, and from 0.4 to 0.7, measured at a temperature of 60xc2x0 C. and a frequency of 10 Hz. EVA in the claimed form possesses good initial bond strength to paint and good paint compatibility. The high ultimate bond strength to paints and glass surfaces, although it permits highly reliable bonding, nevertheless necessitates a disproportionately great effort when removing the film, which may adversely affect the health of persons for whom the demasking is a daily task. Moreover, because of the high bond strength, tearing during the demasking operation is a frequent occurrence, and means that the film cannot be removed in one piece.
Those nontransparent, white films specified in the prior art, however, cannot be used for window protection, since the masked vehicles must be moved and arranged a number of times in the course of their distribution, and an unobstructed view through the windows is vital.
There have been numerous descriptions of window films, which alter the properties of the windows to which they are stuckxe2x80x94they do this, for example, by darkening shading, IR absorption for heat insulation, or as antireflection coatings. U.S. Pat. No. 5,925,453, for example, describes a window film which is light-reflecting and IR-absorbing and can be stuck to the inside of curved auto windows. Preferred candidates as backing materials, which may also be blended with the IR absorbent, include polyolefins such as polyethylene and polypropylene, polyvinyl chloride and polyesters such as polyethylene terephthalate and polybutylene terephthalate. Possible adhesive compositions cited are self-adhesive compositions, including synthetic rubbers such as styrene-butadiene rubber, polyisobutylene, styrene block copolymers and polyethylene-vinyl acetate, and also heat- and moisture-activable compositions. The film described, however, has no protective function, especially since it is applied from the inside to the concave glass surfaces. Articles of this kind are normally conceived for permanent bonding and must therefore meet different key requirements.
In the case of a protective film for automobile glass, particular value must be placed not only on outstanding transparency but also on easy, fold-free and rapid applicability and on a balanced bond strengthxe2x80x94i.e., a bond strength which is not so weak that the film is detached under the influence of wind or slipstream, or even detaches by itself, but also not so strong that it is impossible to remove the film again without excessive effort or even tearing. Moreover, the film must withstand outdoor weathering periods, typical for its application, of approximately 6 months without becoming brittle, becoming dull, or leaving residues of adhesive composition on removal. The abovementioned requirements place stringent demands on film and adhesive composition and on the tailoring of the two components to one another.
Beiersdorf A G, Hamburg, Germany, has for many years sold as Article 4604 PV1 a transparent protective film which is used, inter alia, for window protection but which in many respects does not meet customer requirements of transparency, weathering stability and bonding reliability for this utility. The aforementioned article comprises a polyethylene backing and is coated with an acrylate self-adhesive composition. The film is slightly cloudy, which manifests itself as highly disruptive especially when light is incident from the sides. After a few weeks or outdoor weathering, the film becomes brittle under the effect of UV light, so that it can no longer be removed in one piece and the adhesive composition remains extensively on the glass. Furthermore, the film loses its adhesion properties at temperatures above 60xc2x0 C., which are easily achieved under insolation, whereupon shrinkage-related folds are produced.
It is an object of the invention to provide a surface protection film for automobile glass, especially windshields, and other sensitive surfaces which does not exhibit the above-described disadvantages of the prior art, or not to the same extent. In particular, the surface protection film should possess not only a pronounced initial bond strength but also a balanced ultimate bond strength and an extremely high transparency, and also weathering stability.
This object is achieved by means of a surface protection film as laid down in the main claim. The subclaims relate to advantageous developments of the surface protection film.
The invention accordingly provides the construction of a surface protection film comprising a transparent backing of polypropylene or propylene-ethylene copolymer, which comprises a light stabilizer, and a self-adhesive composition of polyethylene-vinyl acetate (EVA) having a VA fraction of from 40 to 80% by weight and a melt index MFI in accordance with ISO 1133 (A/4) of from 0.5 to 25 g/10 min at 190xc2x0 C. and 2.16 kg, which is blended with from 0 to 35% by weight of a polyether of the form
XOxe2x80x94[(CH2)4xe2x80x94O]xe2x80x94Yxe2x80x83xe2x80x83(1)
or
xe2x80x83XOxe2x80x94[(CH(CH3)xe2x80x94CH2xe2x80x94O]xe2x80x94Yxe2x80x83xe2x80x83(2),
having a weight-average molecular weight Mw of from 200 to 100,000 g/mol, X and Y being selected from the group consisting of Hxe2x80x94, (CnH2n+1)xe2x80x94 where n is from 1 to 20, CH2xe2x95x90CHCOxe2x80x94, CH3CH(NH2)CH2xe2x80x94, 2,3-epoxypropyl-, C6H5xe2x80x94COxe2x80x94, CH2xe2x95x90C(CH3)xe2x80x94COxe2x80x94.
In one preferred embodiment, the surface protection film comprises an unoriented film of from 30 to 120 xcexcm in thickness, preferably from 35 to 80 xcexcm in thickness, comprising a random propylene-ethylene copolymer having an ethylene fraction of from 2 to 10% by weight, preferably from 4 to 8% by weight, and more than 0.3% by weight, preferably more than 0.5% by weight, of a light stabilizer.
The adhesive composition comprises polyethylene-vinyl acetate having a VA fraction of from 40 to 80% by weight, preferably from 45 to 70% by weight, and an addition of (1) or (2) of from 0 to 35% by weight, preferably from 0 to 20% by weight, applied preferably at a rate of from 8 to 50 g/m2, with particular preference from 10 to 30 g/m2.
Between adhesive composition and film there may be a primer comprising EVA having a VA fraction of from 20 to 50% by weight, preferably from 25 to 40% by weight, in a thickness in particular of from 5 to 20 xcexcm, preferably from 5 to 15 xcexcm, which is coextruded with the backing layer. In this preferred form, the film is wound up in its final form directly after coating. The proposed primer possesses outstanding transparency, both alone and in conjunction with the film.
The use of the claimed propylene-ethylene copolymers combines a surface smoothness and transparency that is outstandingly suitable for this utility with great flexibility, so that bonding can take place without folds even to curved surfaces. The mechanical and optical properties of the film are significantly affected not only by the selection of raw materials but also by the production conditions. An advantageous process for the desired properties is the casting process, where the temperature of the chill rolls should be between 10 and 25xc2x0 C. in order to permit extremely fine crystallization.
Under application conditions, the transparency is retained, even after using the windshield wipers and after 6 months in a Florida climate, to such a degree that the vehicle can still be removed without hazard.
Particularly suitable means of ensuring a long life of the film material, in a manner compatible with the application, are HALS light stabilizers such as, for example, dimethyl succinate polymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinoethanol (CAS No. 65447-77-0), bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate (CAS No. 52829-07-9) or poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyl][[(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]] (CAS No. 70624-18-9).
The claimed adhesive composition based on EVA, with and without polyether addition, is highly suitable for glass surfaces, since it possesses the property of flowing out flush with glass and of producing a virtually invisible phase boundary between glass and composition. The EVA in the form described forms the polymer framework of the adhesive composition, with a moderately pronounced tack and a rapidly increasing bond strength thereon, which attains its ultimate value within a few days, this value being achieved more quickly under the action of heat.
Since the polymer framework in question is chemically uncrosslinked, and on the basis of its monomer proportion is only very slightly crystalline, the molecular weight, which correlates directly to the MFI, adopts a decisive position in respect of the cohesiveness of the adhesive composition. An MFI of from 1 to 5 has proven to be a favorable value. The addition of an EVA fraction having an MFI of up to 25, however, may contribute to improving the flow properties if the adhesive composition is to be applied from the melt or coextruded together with a backing.
The optional addition of the polyethers described has the effect of reducing bond strength at the same time as maintaining the required initial tack, the aging stability, the transparency, the health and environmental acceptability, and the freedom of the overstuck glass surfaces from deposits after demasking, if the ultimate bond strength of the adhesive film is perceived to be too high. The unwind behavior of the protective films, wound up into rolls, may also be reduced by the addition of the above-described polyethers, relative to the straight EVA composition. Depending on the fraction of the added polyether, the bond strength reducing effect may be regulated steplessly.
The adhesive composition may be processed either from solution or from the melt, i.e., as a hot-melt, or by coextrusion. From 10 to 30 g/m2 is an advantageous application rate. It ensures a sufficient cushion of composition for said composition to flow out flush with the surface, without there being any notable residues of composition.
For the very high optical requirements imposed on a protective film for windows, or glass generally, fields of use for which the surface protection film of the invention is particularly suitable, it is absolutely necessary, directly after coating, to roll up the film in its final form without folds or bubbles. Subsequent rewinding is highly deleterious to the optical properties, since folds and bubbles impress themselves irreversibly into the film of adhesive composition. Even intermediate release media such as siliconized paper are unsuitable, since the always slightly rough structure is impressed into the cushion of adhesive composition and causes the product to become matt.
The surface protection film of the invention for automobile glass windows in particular is described below in a preferred embodiment on the basis of a number of examples of surface protection films, without wishing thereby to restrict the invention in any way whatsoever. Also given are two comparative examples featuring unsuitable surface protection films.